Production has been increased above the original design capacity, principally in older continuous digesters, so much that the conventional digester wash at the bottom of the digester has essentially been eliminated. It is often the case in these overloaded digesters that the dilution factor at the bottom of digester is 0, and in certain extreme cases it may be negative. The dilution factor is the factor that specifies the quantity of washing or dilution liquid that is added at the bottom of the digester relative to the current quantity of cooking fluid in the digester. For a dilution factor of 2.0, as is often desired, 2.0 cubic meters of washing or dilution liquid is added at the bottom of the digester per tonne of pulp (2.0 m3/ADT).
Also new digester plants are designed such that the greater part of the digester is used as cooking zone, such that a longer retention time in the cooking process is obtained, which allows reduction of the required cooking temperature to achieve the H factor necessary for delignification. A longer retention time and lower cooking temperature are beneficial for the strength and yield of pulp, since the cellulose is broken down to a lesser degree, and they also give better control of the cooking process.
Large digesters with capacities of over 4000 tonnes of pulp per day have extremely large diameters, greater than 12 meters, at the bottom of the digester, and this means that it becomes extremely difficult to establish a good displacement of the free fluid from between the softened chips by the addition and withdrawal of washing or dilution liquid through the wall of the digester and arrangements having central pipes.
The conventional technology for adding washing or dilution liquid through vertical and horizontal nozzles in the wall at the bottom surface of the digester often leads to the formation of flows or a film of liquid along the inner wall of the digester shell down towards the outlet. These flows of low temperature with washing or dilution liquid that has a relatively lower temperature can often be detected on the walls of transfer lines to subsequent storage towers or washing equipment, and in certain cases these cold flows are held intact until the inlet of the storage tower or washing equipment.
A known wash is shown in FIG. 1D at the bottom of the digester where washing liquid is added through a central pipe, arranged directly above the bottom scraper, and where expelled cooking fluid is withdrawn from the surrounding wall of the digester wall. The technology corresponds to that revealed in, for example, U.S. Pat. No. 3,475,271. A variant with several displacement stages is revealed in U.S. Pat. No. 4,213,822. One disadvantage here is that a large part of the bottom section of the digester is used for digester washing.
FIG. 1E shows known dilution technology at the bottom of the digester in which dilution liquid is added at the bottom of the digester, typically through nozzles from a source NH/NV, and where the pulp is dewatered in a subsequent dewatering arrangement 8 in the outlet line 12. The technology corresponds to that revealed in, for example, SE204236. One disadvantage here is that the dilution gives a limited wash, since the filtrate obtained from the dewaterer is recirculated as dilution liquid.
U.S. Pat. No. 3,807,202 reveals a variant of the wash of defibrated pulp at the bottom of towers. A stationary internal distribution space is arranged in this case in the outlet tap, with a surrounding stationary strainer. Washing liquid is added through the central distribution space and expelled liquid is withdrawn through the surrounding strainer. This solution concerns the washing of defibrated pulp, and does not concern the displacement of liquid from softened chips. One disadvantage here is that the strainer and the distributor space are stationary, and where the defibrated pulp must pass through a narrow gap between them. It is easy for the strainer to become clogged by fibre material and the washing process loses its effect.
A further displacement wash of defibrated pulp is revealed in U.S. Pat. No. 6,272,710 and U.S. Pat. No. 6,553,593, where the pulp is divided into thin streams through a rectangular space under atmospheric conditions. This solution is not suitable for a displacement wash of softened chips at full cooking pressure.
The pressure drop that is generated across the blow-valve gives a defibrating effect for the cooked softened chips such that the fibres are released to a greater degree and the pulp can be better washed in a subsequent washing process, preferably a pressure diffuser arranged directly after the digester. It is desired to implement an displacement wash at the relevant process position before the blow-valve between the softened but as yet not defibrated fragments of chip such that the free liquid between the fragments of chip can be exchanged from a free liquid, typically consumed cooking fluid or black liquor, with a high content of dissolved organic material, principally but not exclusively lignin, to a cleaner liquid with a lower content of dissolved organic material.
After defibration in the exchanged cleaner liquid, organic material that was bound in the softened chips can more easily dissolve and the total washing efficiency from the subsequent wash can be considerably improved.
A first purpose of the invention is to achieve an improved displacement wash of the digested and softened chips before defibration of the chips takes place across the blow-valve.
A second purpose is to be able to install this displacement wash in already existing parts of the equipment at the digester plant, such that no further equipment or components are required. The displacement washer can, when installed in new digester plants, be achieved at very low additional cost since it is only necessary to exchange one pulp line for a displacement washer.
A third purpose is to be able to offer, by the rebuilding of existing equipment, an increase in the dilution factor in overloaded digesters, where the production has been increased from the original design capacity so much that the dilution factor has been severely reduced, and in certain cases even eliminated.
The method according to the invention relates to improvement of a wash after completed digestion in a continuous digester for the production of cellulose pulp. Softened chips are fed in the method out from the bottom surface of the digester after having passed a final strainer section in the bottom of the digester. The softened chips are fed out under the influence of a bottom scraper arranged at the bottom of the digester and through a bucket shaped outlet tap at the bottom of the digester and further to an outlet line connected to the outlet tap. Finally, the softened chips pass through a blow-valve arranged in the outlet line, across which blow-valve a pressure drop of at least 0.5 bar and at most 3-5 bar has been established. What is characteristic for the method according to the invention is that the softened chips are exposed to a displacement wash after they have passed the outlet tap, and which displacement wash has been established in the flow of digested softened chips through the outlet line before the softened chips are defibrated by the pressure drop across the blow-valve.
The displacement wash is preferably established between two opposing walls of the outlet line, where one wall has nozzles for the addition of washing fluid and the second opposing wall has strainer surfaces for the withdrawal of liquid.
The displacement wash in the outlet line through the addition and withdrawal in the opposing walls preferably exchanges between 0.1 and 2 cubic meters of liquid per tonne of pulp (ADT) for each 2 meters of outlet line.
In order to minimise the risk of clogging of the section of wall that has strainer surfaces for the withdrawal of liquid, it is appropriate that the displacement wash is established, in at least one module through the addition and withdrawal across the opposing walls, such that this displacement wash changes its direction of displacement after a predetermined time, such that first wall section add washing fluid during a first period of time and then in a subsequent second period of time withdraw liquid, with the inverse functionality taking place in the second wall section. This alternation of the direction of displacement may also take place in a manner based on feedback, through detection of the pressure drop across the bed of chips.
The arrangement is intended for the improvement of a wash after a completed digestion process in a continuous digester for the production of cellulose pulp, in which digested softened chips are fed out from the bottom surface of the digester after having passed a final strainer section in the bottom of the digester and subsequently fed out under the influence of a bottom scraper arranged in the bottom of the digester and thereafter through a bucket shaped outlet tap at the bottom of the digester and onwards to an outlet line (12) connected to the outlet tap, before the softened chips pass through a blow-valve (4) arranged in the outlet line, across which blow-valve a pressure drop of at least 0.5 bar and at most 3 bar has been established. The chips at this position before the blow-valve have been softened and maintain their structure essentially as a bundle of softened fibres. The only noticeable mechanical influence to which the softened chips are exposed is that of the bottom scraper. The arrangement at this position is characterised in that it is constituted by a module of the outlet line before the blow-valve is provided with at least one first axially running section of wall with nozzles for the addition of washing fluid and at least one second section of wall with strainer surfaces for the withdrawal of expelled cooking fluid, where these sections of wall are located at diametrically opposite sides of each other in the inner surface of the outlet line.
It is appropriate that the module in the outlet line has a length of between 2 and 6 meters. Since full cooking pressure has been established in the pulp at this position, also the module must be designed as a pressure vessel, and it is for this reason appropriate that the module be constituted by a tubular pressure vessel, in which pressure vessel the wall sections for the addition and withdrawal of washing fluid are arranged as exchangeable plates.
In order to achieve better displacement washing through thin flows of the softened chips, the module of the outlet line before the blow-valve can be provided with at least one third section of wall that runs in an axial direction and that has nozzles for the addition of washing fluid, and at least one fourth section of wall that runs axially and that has strainer surfaces for the withdrawal of expelled cooking fluid, where these sections of wall are arranged such that sections of wall with nozzles and a sections of wall with strainer surfaces are located arranged facing each other in the inner surface of the outlet line.
By designing the arrangement as a module, increased displacement washing can be easily installed by arranging at least two modules of the outlet line in series. In the case of this installation, at least one module of the outlet line can be arranged in a horizontally running part of the outlet line and at least one module of the outlet line can be arranged in a vertically running part of the outlet line.